Hello, if I have understood
The integration of Fm with PS is a level grafo, when the integration is activated, elements of imputation for the grafo is active, then you can define the origin of the funds in the grafo.
In requisition purchase (me51n) order requests must be allocated to the grafo this take the fund source has been assign.
http://help.sap.com/saphelp_46c/helpdata/es/37/113274f6c411d1953a0000e835339d/content.htm
I hope helped
Regards